In an optical communication system, when performing high-capacity long-distance transmission at high speed, nonlinear optical phenomena such as wavelength dispersion and polarization mode dispersion (PMD) become prominently manifested, causing waveform distortion which, in turn, causes an increase in the error rate of a received signal.
Compensation control must be performed on such waveform distortion at the receiving side of the optical communication system. A received signal sequence estimation method using maximum likelihood sequence estimation (MLSE) technology has been realized as compensation to be performed at an electrical stage.
Maximum likelihood sequence estimation refers to an estimation method of searching for most probable information data that maximizes the likelihood of presently received information data based on previously obtained likelihood (probability) information.
In this case, likelihood refers to a conditional probability that Y has been transmitted in received data X. In other words, likelihood refers to the probability of the occurrence of an event in which Y is transmitted, given that an event in which data X is received has already occurred.
As a conventional technique for maximum likelihood sequence estimation, a technique involving estimating a transmission channel impulse response initial value in a known signal sequence segment to reduce the amount of computation to be performed is proposed in paragraph [0068] and FIG. 1 in Japanese Patent Laid-Open No. H07-095107 (Japanese Patent No. 3560991). In addition, a technique for creating an accurate expected value from a histogram of a playback signal sampling value even when an offset occurs in the playback signal is proposed in paragraph [85] and FIG. 4 in Japanese Patent Laid-Open No. 2003-187533.
With a receiving device that performs bit estimation using maximum likelihood sequence estimation, bit estimation of a received signal is performed using previously obtained likelihood information. In this case, a method is adopted in which a likelihood determined upon bit estimation is fed back to previously obtained likelihood information to update the likelihood information and sequentially generate new likelihood information.
In addition, with a receiving device, during initial activation thereof, the receiving device retains initial likelihood information (an initial value of a likelihood distribution) as a first piece of likelihood information to be used in bit estimation and, based on the initial likelihood information, performs bit estimation and updates likelihood information.
Therefore, in order to perform maximum likelihood sequence estimation with high accuracy, it is required that the transmission state of a signal received by the receiving device is appropriately reflected in initial likelihood information that becomes a basis when performing update processing.
However, with a receiving device that performs conventional maximum likelihood sequence estimation, initial likelihood information is often set as a fixed default value and does not reflect information on the transmission state of the actual network where the receiving device is disposed.
Since using initial likelihood information not reflecting a transmission state means that the first piece of likelihood information for performing bit estimation is erroneous, bit estimation cannot be performed with high accuracy. In addition, updating likelihood information using erroneously judged likelihoods results in errors being accumulated in likelihood information. This problem becomes prominent when there is a significant difference between a waveform of a received signal assumed in order to obtain initial likelihood information and a waveform of an actually transmitted received signal.
FIG. 46 is a diagram illustrating an eye pattern of a received signal. The waveform of an actually transmitted received signal is susceptible to degradation due to waveform distortion and noise which often result in a lower eye aperture ratio. Reducing the error rate of a received signal requires that a transmission state due to such waveform degradation occurring on a transmission channel be appropriately reflected in initial likelihood information to be used in bit sequence estimation. However, conventionally, such transmission states have not been reflected.
Consequently, the deviation between a transmission state of an actual network and an optimum value increases, resulting in a higher error rate. Moreover, in the case of an occurrence of an excessive error, there is even a risk that the receiving device will fail to activate.